Semiconductor wafers, as they come cut from a crystal of doped silicon, and prior to their fabrication as integrated circuits, are typically sorted in accordance with various electrical characteristics. Automated equipment has evolved to accomplish this function, and it is desired that the detection of conductivity-type in the wafer, whether N- or P- type, be provided in such sorting systems.
The detection of wafer conductivity-type is made difficult by the presence of static charge on the semiconductor wafer which normally occurs during automated or other handling of such wafers. The existence of a charge will create in many cases an erroneous indication of conductivity-type. The charge must therefore be deleted from the wafer before measurement of conductivity-type, at least in the area of probing. Charge elimination, due to the exponential character of discharge curves, is a time consuming operation, and even when conducted for an extended period of time will not completely discharge the wafer. Even though apparently fully discharged, charge may spread back into the probed region of the wafer from peripheral regions or from other sources. The presence of any such charge will tend to distort any reading of conductivity-type made. Therefore, a high degree of security is required in wafer discharging in combination with a relatively fast discharge function.
The reading of conductivity type in acceptably discharged wafers is subject to errors or uncertainty probabilities of sufficient magnitudes as to impair the value of automated conductivity-type testing. A major source of such error is the difficulty in getting good rectifying contact to a silicon wafer. Many of these cannot be eliminated from automated conductivity-type testing instrumentation without creating substantial problems in handling speed or equipment expense. Another mechanism is required, however, for increasing the reliability of conductivity-type measurement.